The Bigger Your U-Tube water,.....The More Inaccurite?????

[Flash]

I have a 8 Foot water gauge.
Yes that's 48" up and 48" down. Yes i know that =96" of water U-Tube

I installed a U-Tube that uses gauge oil today,(Thanks again Ray!!!) for a comparison of my water gauge.
Both were adj to show "0"

What i found was kinda shocking!


this is at 10"........notice the water gauge (blue fluid) is 1" off each side, or at only 8" of water


At 20"(which is the max reading for the Gauge oil U-Tube) the water is off by almost 4"(actually 3.5")


What's going on here?

[Brucepts]

Are they "T" together?

[86rocco]

What's wrong is the scale on the manometer with the red oil, red gauge oil is less dense than water so 1" WC of pressure will move the column on the manometer by 1/(specific gravity of the fluid), the scale needs to stretched out to compensate for the lower density fluid. Based on what you've told us, I'm guessing your gauge oil is the typical 0.826 specific gravity fluid so if your water gauge reads 16.5", the gauge red oil would read 16.5/0.826=19.98"

[86rocco]

Here's a somewhat unorthodox way of making a U tube manometer easy to read accurately and without spending a lot of time making a fancy scale, with your 0.826 sp red gauge oil, tilt the whole U-tube over to 37.25°, use a simple yard stick for a scale and read your pressure directly off the yard stick.


BTW, 37.25° is the angle you get when you have a right angle triangle with a rise of 21.79" with a hypotenuse of 36" and 21.79" is the amount of change you see along one leg of a U-tube manometer filled with 0.826 fluid when you apply a pressure of 36" WC.

[coulterracn]

The smaller 20" u-tube manometer shown in Flash's picture is made for Kohler engine service by Dwyer and uses the Dwyer red gage oil. It's not a DIY u-tube manometer.

[Flash]

First off, thanks for all the input!!!!!

To clear up a couple things, ignore my incline to the left, as it its just red food coloring and is not part of my test.

My intentions with this new 20" u-tube was to see how much water varied with temperature compared to a stable fluid(Gauge oil) I was not at all expecting to see this ratio difference.
I was completely shocked at the huge variation.

Bruce yes the 96" and the 20" are tied into the same port on my flow bench.

The smaller 20" u-tube manometer shown in Flash's picture is made for Kohler engine service by Dwyer and uses the Dwyer red gage oil. It's not a DIY u-tube manometer.

So when you bought this gauge, that was the oil that came with it, right.

[coulterracn]

The smaller 20" u-tube manometer shown in Flash's picture is made for Kohler engine service by Dwyer and uses the Dwyer red gage oil. It's not a DIY u-tube manometer.

So when you bought this gauge, that was the oil that came with it, right.

Yes, I sent you an unopened bottle of the red gage oil that came with the manometer.

Ray

[Flash]

So with this U tube using .8XX gauge oil, would that mean that the "one inch" space would be actually .8XX apart? is that how its figured?

[coulterracn]

So with this U tube using .8XX gauge oil, would that mean that the "one inch" space would be actually .8XX apart? is that how its figured?

It should be less than 1", I do not know the exact measurement. I can measure my Dwyer vertical manometer and send that measurement to you.

[86rocco]

So with this U tube using .8XX gauge oil, would that mean that the "one inch" space would be actually .8XX apart? is that how its figured?

It should be less than 1", I do not know the exact measurement. I can measure my Dwyer vertical manometer and send that measurement to you.

Because the fluid has a lower specific gravity than water, one inch of fluid movement on the manometer would correspond to 0.826"WC of pressure and, one inch of pressure would show 1/0.826"= 1.121" of movement on the manometer so, if you've making a scale and you want to measure from the zero point along only one leg of the manometer, you'd need to put your "inch" marks half that distance apart or 0.605 inches apart.